Process for producing silicon-iron thin tapes



Sept. 6, 1966 E. A. FREEMAN ETAL 3,271,202

PROCESS FOR PRODUCING SILICON-IRON THIN TAPES- Filed Dec. 18, 1965 Irvverwkcrs: Eleanor A, Free mam, John .AZl d/er, y Th air At tates This invention relates to silicon-iron .sheet and strip and more particularly to such sheet and strip of up to 6 mils thickness which have improved watt-loss characteristics, particularly under A.C. operating conditions.

In the construction of the many tyms of induction apparatus, magnetic materials having diverse and often special magnetic properties are required. Thin tapes of silicon-iron of up to 6 mils in thickness are especially useful in the construction of magnetic coils or cores. Improvement in the electrical and magnetic operating characteristics of these cores has, in the past, been obtained by utilizing strip material having a preferred grain orientation, especially the cube-on-edge or (110) [001] grain orientation. Such grain oriented thin tapes frequently consist of grains of quite large size, for example on the order of one centimeter and larger and while the magnetic properties are good, the watt-losses are frequently higher than desired.

It is a principal object of this invention to provide a process for producing silicon-iron thin tapes which have small grain sizes and improved watt-loss characteristics compared to large grained materials.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification and drawings.

FIG. 1 is a photomicrograph of a silicon-iron thin tape illustrating the grain structure obtained by existing processes;

FIG. 2 is a photomicrograph of a silicon-iron thin tape showing the grain structure obtained when the strip is processed according to the present invention; and

FIG. 3 is an enlarged photomicrograph of a siliconiron thin tape processed according to this invention but using an atmosphere different from that used on the tape of FIG. 2.

Broadly, the process of the present invention is effected by providing a cold rolled silicon-iron thin tape of up to 6 mils thickness, giving the tape a comparatively low temperature, oxidizing pro-anneal to form a thin coherent oxide film on the surface thereof and then vacuum annealing the strip at temperatures higher than those used in the pre'anneal to obtain a (110) [001] crystallographic orientation.

Considering the processing in more detail, ingots are prepared by vacuum melting high purity iron and silicon together in proportions such that the silicon content of the final alloy falls between 1.5 and 4 percent by weight. The impurity content of the cast alloy should not exceed 0.01 \weight percent as a maximum and preferably should be kept as low as possible, for example not more than about 0.006 weight percent. The normal content for some of the more usual impurities is: carbon 0.001; sulfur 0.001; oxygen 0.001; and nitrogen 0.0005; the remainder being minor metallic impurities which are normally present. There should be no materials included which will form a dispersed second phase in the alloy since the procedure is one in which grain orientation is obtained by a surface energy effect.

Once a cast ingot is obtained, it is hot rolled to 0.25 inch. At this point, the material is annealed at 700 C. to 1200 C. for from about 0.1 to 1.0 hour in dry hydrogen, that is, hydrogen which is substantially nonoxidizing to silicon, viz., dew point no higher than atent 40 F. The slabs are then rolled to about 0.10 inch thickness. The material at this stage is subjected to an annealing at temperatures ranging from about 700 C. to 1200 C. for a time sufiicient to recrystallize the hot worked texture, e.g. about 0.1 to 10 hours in a substantially non-oxidizing atmosphere such as dry hydrogen. It is to be understood that all references to dry hydrogen means hydrogen having a dew point no higher than -40 F. unless specified to the contrary.

The annealed product is then cold worked at least 25 percent in substantially the same direction (e.g. over the range of 40 percent to 99 percent) using intermediate anneals where required when more than one reduction stage is used to arrive at thicknesses ranging up to about 6 mils (0.006 inch). The intermediate anneal, which is also carried out under substantially non-oxidizing conditions, may range in temperature from 700 C. to 1200 C. for a time sufficient to effect recrystallization, such as 0.1 hour to 10 hours.

Following obtention of material of the desired final thickness, the normal procedure for obtaining cube-onedge oriented tapes is to subject it to a final recrystallizing anneal at an elevated temperature in a selected environment and for a sufiicient time to effect secondary recrystallization. Temperatures for the final anneal may range from about 1000 C. to 1350 C. in atmospheres such as hydrogen having a dew point no higher than about F. and vacuum at pressures no higher than l l0*4 mm. of Hg. Lower vacuum pressures are especially advantageous, for example, pressures on the order of 6 l0- A more detailed discussion of the general processing outlined above can be found in U.S. Patent No. 3,105,782 issued to John L. Walter, October 1, 1963, and assigned to the same assignee as the present application. The size of the grains in silicon-iron thin tapes produced according to the procedure just outlined can be seen by reference to FIG. 1 of the drawings, this material consisting of 3.2 percent silicon, not more than 0.010 incidental impurities, balance substantially all iron.

In accordance with the present invention, it has unexpectedly now been found that the thin tapes can be given a pre-anneal at the termination of cold rolling and before the advent of final recrystallizing anneal which will result in grain sizes that are significantly smaller. Specifically, the thin tapes are given a comparatively low temperature pre-anneal between about 250 C. and 700 C. in an environment containing a small amount of oxygen. The purpose of this anneal is to form a thin coherent oxide film on the surface of the tape before it is subjected to the 1000 C. to 1350 C. final recrystallizing and grain growth anneal. The film is kept thin, the thickness normally not exceeding 3000 A. A heavy or gross oxide coating is totally ineffective in carrying out the process. Generally, if the tape is annealed in an environment containing a large amount of oxygen, for example air, then the pre-annealing temperature will be kept at the lower end of the previously stated range. Conversely, in environments containing limited amounts of oxygen, for example vacuum, temperatures at the upper end of the pre-annealing range would be utilized.

Additionally, the annealing time is much shorter when a large amount of oxygen is present and comparatively longer when a small amount is present. Normal annealing times range from about 1 minute to 15 minutes depending upon the temperature and oxygen environment utilized.

FIG. 2 of the drawings shows the grain size present in tape of the same composition as that of the body of FIG. 1 in which the body was given a pre-anneal at a temperature of 700 C. for 10 minutes in a vacuum of 1x 10* mm./Hg. This strip was given a final recrystallizing anneal at 1250 C. following the pre-anneal in a vacuum of 1X10" mm. of Hg. It is apparent upon comparing the grain sizes of the two bodies shown in these figures that the grains in the body of FIG. 2 are smaller by a factor of 3 to 4 than the grains in FIG. 1.

Similarly, the size of the grains in the body shown in FIG. 3 are significantly smaller than those of the grains in FIG. 1. The body of FIG. 3 was produced by preannealing cold rolled strip of final thickness in air at 350 C. for 5 minutes; here, the time being less and the temperature lower because of the comparatively larger percent of oxygen present in the annealing environment. By virtue of the smaller grain sizes present in the bodies processed according to this invention, the material has lower losses (measured in watts per pound) in magnetic cores used for AC. applications.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The process for producing high purity silicon-iron thin tapes comprising providing a cold rolled tape of up to 6 mils thickness which compositionally consists of from about 1.5 to 4.0 weight percent silicon, remainder substantially all iron and containing not more than about 0.010 weight percent incidental impurities, annealing the tape at a temperature and for a time sufficient to form integral iron oxide film of up to 3000 A. thickness on the surface thereof, and subjecting the annealed tape to a final anneal in a vacuum no higher than about 1 10 mm./Hg at temperatures of from about 1100 C. to 1350" C. to develop the (110) [001] crystallographic orientation.

2. A process as defined in claim 1 wherein said annealing is effected at temperatures ranging from 250 C. to 700 C.

3. A process as defined in claim 2 wherein said annealing is carried out for times ranging from about 1 to 15 minutes.

References Cited by the Examiner UNITED STATES PATENTS 2,303,343 12/1942 Engeletal. 14s 111 2,307,391 1/1943 Coleetal 14s 110 2,378,321 6/1945 Pakkala 14s 110 2,529,373 11/1950 Campbell et al 14s 110 3,105,781 10/1963 Walter 14s 111 3,125,473 3/1964 Schneideretal 14s 111 DAVID L. RECK, Primary Examiner.

N. F. MARKVA, Assistant Examiner. 

1. THE PROCESS FOR PRODUCING HIGH PURITY SILICON-IRON THIN TAPES COMPRISING PROVIDING A COLD ROLLED TAPE OF UP TO 6 MILS THICKNESS WHICH COMPOSITIONALLY CONSISTS OF FROM ABOUT 1.5 TO 4.0 WEIGHT PERCENT SILICON, REMAINDER SUBSTANTIALLY ALL IRON AND CONTAINING NOT MORE THAN ABOUT 0.010 WEIGHT PERCENT INCIDENTAL IMPURITIES, ANNEALING THE TAPE AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO FORM INTEGRAL IRON OXIDE FILM OF UP TO 3000 A. THICKNESS ON THE SURFACE THEREOF, AND SUBJECTING THE ANNEALED TAPE TO A FINAL ANNEAL IN A VACUUM NO HIGHER THAN ABOUT 1X10-4 MM./HG AT TEMPERATURES OF FROM ABOUT 1100*C. TO 1350*C. TO DEVELOP THE (110) (001) (001) CRYSTALLOGRAPHIC ORIENTATION. 